1. Technical Field
This invention relates generally to gas turbine engines and particularly to a mounting arrangement for gas turbine engine blades such as fan blades.
2. Background Information
Turbofan gas turbine engines such as those which power aircraft employ a multiplicity of fan blades attached to a hub mounted on the forward (upstream) end of one of the engine shafts. Typically, such fan blades are provided with a radially outer airfoil shaped portion and a radially inner root portion typically having a dovetail shape. The dovetail shaped root portion is received within a slot which conforms thereto in the fan hub. For ease in attaching the fan blade to the hub by sliding the root portion into the slot and for removal of the blade from the hub by sliding the root portion of the blade out of the slot, the slot is usually slightly larger than the dovetail root portion. This difference in dimensions between the root portion of the fan blade and the slot in the hub results in a clearance between the root portion and the slot. Under normal engine operating conditions when the engine's rotor is spinning at high speed (several thousand rpm) centrifugal force acting on the fan blade causes the blade to be held tightly in the hub slot.
However, when the engine is not in use, wind acting on the fan blades can cause the engine's rotor to slowly turn. This slow turning of the engine rotor in response to winds acting on the fan blades is referred to as windmilling. There is very little centrifugal force acting on the fan blades during such windmilling due to the low rotational speed of the engine rotor in response thereto and thus, the fan blade roots are not tightly held within the conforming slots in the fan hub, resulting in movement between the fan blade root portions and the hub slots in which they are received. This movement of the fan blade root portions within the slots, if unchecked, can result in damage to the fan blade root portions and the slots due to galling and fretting of the surfaces of the root portions and the slots. To minimize such galling and fretting, it has been a practice to employ spacers between the radially innermost end of the root portion and the adjacent portion of the slot which receives the root portion to prevent movement between the blade and the hub during windmilling of the engine's rotor. In some cases, such spacers actually resiliently bias the root portion and thus the entire blade radially outwardly to tightly secure the blade root portion within the hub slot. Such prior art blade root spacers have taken the form of relatively complex metallic configurations and elastomeric materials secured between rigid clamping members which adjustably compress the elastomeric material to control the elasticity thereof. The complexity and weight of such prior art blade root spacers adds to the cost and weight of the engine, thus detracting from the efficiency thereof. Furthermore, the complexity of such spacers detracts from the ease with which the fan blades are assembled to the hub during engine assembly and removed from the hub for engine maintenance.
Accordingly, it will be appreciated that a need exists for a simple, lightweight and economical means for minimizing movement of fan blade roots within fan hub slots under conditions such as windmilling and the like.